1. Field of the Invention
The invention relates generally to apparatus and methods relating to the application of coatings, and more particularly to a two-stage kinetic energy spray device.
2. Description of Related Art
Thermal spraying is generally described as a coating method in which powder or other feedstock material is fed into a stream of energized gas that is heated, accelerated, or both heated and accelerated. The feedstock material becomes entrapped by the stream of energized gas, from which the feedstock material receives thermal and/or kinetic energy. This absorbed thermal or kinetic energy softens and energizes the feedstock. The energized feedstock is then impacted onto a surface where it adheres and solidifies, forming a relatively thick thermally sprayed coating by the repeated cladding of subsequent thin layers.
Conventional cold spray devices either inject the powder feedstock before or after the throat of a Laval type convergent/divergent nozzle. When the feedstock is injected before the nozzle it is typically performed in an axial orientation at or near the beginning of the convergent nozzle section, and the powder feedstock is heated and accelerated through the Laval nozzle. This allows the particles to have a relatively uniform acceleration profile, however the particles are also subjected to the same elevated gas temperatures that are required for optimal performance of the Laval nozzle since the gas velocity is a function of the square root of the gas temperature. These optimal temperatures, typically in excess of 500 C., pre-soften the powder feedstock which can and often results in the powder sticking to the nozzle walls at the throat. Another limitation is that the particle temperature cannot be independently controlled since the gas temperature directly controls both the particle velocity and the particle temperature.
Injection of the feedstock after the throat is performed radially anywhere along the divergent section of the nozzle. This method has the advantages of not loading the nozzle throat with powder as well as providing some independence to the particle temperature because the powder feedstock is injected when the gas is expanding and cooling rapidly. A significant disadvantage is that the powder feedstock is injected into a supersonic gas stream and the difference in velocity between the gas and the particles results in considerable and significant drag heating and energy waste. The result is that a measureable portion of the kinetic gas energy is transferred into heat both in the gas and the particles. Accordingly, the greater the difference in velocities between the particles and the gas, the wasted kinetic energy increases exponentially.
It has been previously recognized that, in the case of some thermal spray applications, injecting feedstock axially into an energized gas stream presents certain advantages over other feedstock injection methods. Typically, feedstock is fed into a stream in a direction generally described as radial injection. In other words, in a direction that is generally perpendicular to the general direction of travel of the gaseous stream. Radial injection is commonly used as it provides an effective means of mixing particles into an effluent stream and thus transferring the energy to the particles in a short span. This is the case with plasma where short spray distances and high thermal loading require rapid mixing and energy transfer for the process to apply coatings properly. Axial injection can provide advantages over radial injection due to the potential to better control the linearity and the direction of feedstock particle trajectory when axially injected. Other advantages include having the particulate in the central region of the effluent stream, where the energy density is likely to be the highest, thus affording the maximum potential for energy gain into the particulate. Still further, axial injection tends to disrupt the effluent stream less than radial injection techniques currently practiced.
Accordingly, in many thermal spray process guns, axial injection of feedstock particles is preferred for the injection of particles, using a carrier gas, into the heated and/or accelerated gas simply referred to in this disclosure as effluent. The effluent can be plasma, electrically heated gas, combustion heated gas, cold spray gas, or combinations thereof. Energy is transferred from the effluent to the particles in the carrier gas stream. Due to the nature of stream flow and two phase flow, this mixing and subsequent transfer of energy is limited in axial flows and requires that the two streams, effluent and particulate bearing carrier, be given sufficient time and travel distance to allow the boundary layer between the two flows to break down and thus permit mixing. During this travel distance, energy is lost to the surroundings through heat transfer and friction, resulting both in lost efficiency and the slowing down of the mixed-flow. Many thermal spray process guns that do utilize axial injection are then designed longer than would normally be required to allow for this mixing and subsequent energy transfer.
These limitations to mix the particulate bearing carrier and effluent streams becomes even more pronounced when the particulate-bearing carrier fluid is a liquid, and, in many cases, they have prevented the effective use of liquid feeding into axial injection thermal spray process guns. For liquid injection techniques the use of gas atomization to produce fine droplet streams aids in getting the liquid to mix with the effluent stream more readily to enable liquid injection to work at all. However, this method still requires some considerable distance to allow the gas and fine droplet stream and effluent stream to mix and transfer energy. This method also produces a certain amount of turbulence in the stream flows.
Attempts at promoting mixing such as introduction of discontinuities and impingement of the flows also produces turbulence. Radial injection, commonly used with thermal spray processes such as plasma to ensure mixing in a short distance also produces turbulence as the two streams intersect at right angles. In fact, most acceptable methods of injection that promote rapid mixing currently use methods that deliberately introduce turbulence as the means to promote the mixing. The turbulence serves to break down the boundary layer between the flows and once this is accomplished mixing can occur.
The additional turbulence often results in unpredictable energy transfer between the effluent and particulate bearing carrier stream because the flow field is constantly in flux. This additional turbulence produces variations within the flow field that affect the transfer of energy. Turbulence represents a chaotic process and causes the formation of eddies of different length scales. Most of the kinetic energy of the turbulent motions is contained in the large scale structures. The energy “cascades” from the large scale structures to smaller scale structures by an inertial and essentially inviscid mechanism. This process continues creating smaller and smaller structures which produces a hierarchy of eddies. Eventually this process creates structures that are small enough that molecular diffusion becomes important and viscous dissipation of energy finally takes place. The scale at which this happens is in the Kolmogorov length scale. In this manner the turbulence results in conversion of some of the kinetic energy to thermal energy. The result is a process that produces more thermal energy rather than kinetic energy for transfer to the particles, limiting the performance of such devices. Complicate the process by having more than one turbulent stream and the results are unpredictable as stated.
Turbulence also increases energy loss to the surroundings because turbulence results in loss of at least some of the boundary layer in the effluent flow field and thus promotes the transfer of energy to the surroundings as well as frictional affects within the flow when flows are contained within walls. For flow in a tube the pressure drop for a laminar flow is proportional to the velocity of the flow. In contrast, for turbulent flow the pressure drop is proportional to the square of the velocity. This gives a good indication of the scale of the energy loss to the surroundings and internal friction.
The original design of a cold spray gun was patented as U.S. Pat. No. 5,302,414, utilizing a single convergent/divergent nozzle to accelerate a stream of particles injected into a flow of gas that is then passed through the nozzle. The gas flow was heated to further increase the velocity. This velocity increase of the gas was preferably a result of the relationship that gas velocity is proportional to the square root of the gas temperature.